Stalagmites and stalactites are made of calcium carbonate, and formed in caves by dripping water. This magnificent example is 20 cm wide and 28 cm high, a generous gift from the family of Bill de Leeuw. Photograph by Peter B Tzannes, 2004

Travertine is a noncrystalline form of calcite, opaque, massive, and colorful. It is formed during the evaporation of water. This piece was found near Diakofti. 9.5 cm. Photograph by Peter B Tzannes, 2004

Graphite is one of the world’s softest minerals, and forms in masses or sheets. It is opaque black, soft and light, and has a submetallic luster. 6 cm, northern Kythera. Photograph by Peter B Tzannes, 2004

Aragonite has the same chemical composition as calcite, but crystallizes in a different pattern and doesn’t cleave as distinctly. This piece, 4.5 cm, was found near Fratsia. Photograph by Peter B Tzannes, 2004

Mica has a highly metallic luster, and when it forms in sheets over a host rock it looks as though the rock has been covered in metal foil. 7 cm, found in northwestern Kythera. Photograph by Peter B Tzannes, 2004